Since in an exhaust gas to be exhausted from an internal combustion engine such as a diesel engine, there is contained large amounts of particulates (particulate matter) whose main component is carbon to be the cause of environmental pollution, it is general to mount a filter for collecting (filtering) particulates in these exhaust systems.
Normally, as a filter to be used for such purpose, as is illustrated in FIGS. 6A and 6B, used is a plugged honeycomb structure 31 including a honeycomb structure 32 in which a plurality of cells 39 in communication between two end faces are defined and formed by a porous partition wall 37, and plugged parts 40 disposed in checker board patterns that are complementary between one end face side and the other end face side of the honeycomb structure 32 so that either one of two open ends of each of the cells 39 is plugged.
An exhaust gas flows in an internal part from one end face 33 of a filter that is formed of such plugged honeycomb structure 31, after having removed particulates and the like that are contained in the gas, and flows out from the other end face 35. In specific, first an exhaust gas flows in a cell 39b which end portion is not plugged at one end face 33 of this filter and which end portion is plugged at the other end face 35, penetrates a porous partition wall 37, moves to a cell 39a which end portion is plugged at one end face 33 and which end portion is not plugged at the other end face 35, and exhausted from this cell 39a. Furthermore, on this occasion, the partition wall 37 functions as a filter layer, and particulates in the gas are caught at the partition wall 37 and accumulated on the partition wall 37.
As a method of manufacturing the above-mentioned plugged honeycomb structure, known is a method in which after a transparent sheet and the like is adhered onto an end face of the honeycomb formed body and the end face is imaged, the position of each cell is recognized by image processing of the image having been shot, and based thereon, punching is made by e.g., laser machining in the position corresponding to the cell to be plugged of the sheet. Thereafter, an end portion of the honeycomb formed body is immersed in a plugging slurry that is slurry materials of a plugging part, and the plugging slurry is filled in the end portion of the cell that should be plugged, to form a plugged part (refer to, for example, Patent Document 1).
In this manufacturing method, as mentioned above, recognition of the position of a cell is made by image recognition. In specific, an image of an end face of a honeycomb formed body having been imaged by e.g., a CCD (charge-coupled device) is binary-processed, and in the case in which a mass of black image in the images of having been binary-processed has a predetermined cell shape (for example, a quadrilateral shape), or reaches a predetermined rate (for example, not less than 80%) of a predetermined cell area, this image is determined to be a cell and the position thereof is recognized. Incidentally, an image that does not have a predetermined cell shape or that does not reach a certain rate of a predetermined cell area is determined to be a foreign substance such as dust that sticks to the end face of the honeycomb formed body, and punching is arranged not to be erroneously made in this position.
However since among cells, a cell that is positioned at the most outer peripheral portion of a honeycomb formed body (cell adjacent to the outer peripheral wall of the honeycomb formed body), as compared to the other cells, is shaped to be partially deformed, as well as has a smaller area, in the above-mentioned image processing, there are some cases in which its position is not recognized as a cell. Thus, even if the cell is the one that has to be plugged, no punching is made in the position corresponding to the cell of the sheet and a plugging slurry is not filled in the cell, so that as a result, a plugged part is not formed and a cell which both end portions are left to be open will remain.
In such cells which both end portions remain to be open, since most particulates in an exhaust gas having flowed therein are not caught, but pass through the filter, the presence of the cell will cause the reduction of the performance as a filter.
Incidentally, a honeycomb formed body to be supplied in manufacturing a plugged honeycomb structure by forming a plugged part is normally formed using a die for extrusion forming as illustrated in FIG. 4. This die 41 for extrusion forming includes: a honeycomb forming die 2 having the structure that there is provided at the surface a groove-like slit 4 in a form of a cell block 5 and there is provided at the backside a clay introduction hole (back hole) 6 in communication with the slit 5; and a frame-shaped presser plate 3 that is disposed at the outer peripheral portion of the honeycomb forming die 2 and regulates the shape and dimension of the honeycomb formed body, and forming clay having been supplied from the backside is introduced into the honeycomb forming die 2 from the clay introduction hole 6 and extruded from the surface of the honeycomb forming die 2 through the slit 4, to be a honeycomb formed body (refer to, for example, Patent Document 2).
As mentioned above, in comparison with the other cells, a cell that is positioned at the most outer peripheral portion having a partly deformed shape (hereinafter, in some cases, it may be referred to as “imperfect cell”), among cell blocks of the honeycomb forming die 2, in the extrusion direction of the forming clay, is formed by the clay having passed around a cell block 5′ which end face 5a′ is positioned to be overlapped with an inner circumferential surface 3a of a presser plate 3. In the conventional die 41 for extrusion forming as illustrated in FIG. 4, all the cell blocks 5 and 5′ at least a part of which end faces 5a and 5a′ are resided inside the inner circumferential surface 3a of the presser plate 3 have the same length, that is, it is general that the end faces 5a and 5a′ of these cell blocks 5 and 5′ are present in the same plane.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2001-300922
[Patent Document 2] Japanese Patent Application Laid-Open No. 2003-285308